collect! modifies node attributes use collect

I don't think modification is what you want here.
TemplateError: Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'

```
Template Context:

-----------------
on line #9
  7: #	Separate multiple interfaces with spaces, e.g. "eth0 eth1".
  8: <% unless @interfaces.empty? -%>
  9: <%= @var.upcase %>="<%= @interfaces.collect! {|i| i }.join(" ") %>"
 10: <% end -%>

[2013-03-23T15:34:22+00:00] ERROR: Running exception handlers
[2013-03-23T15:34:22+00:00] ERROR: Exception handlers complete
[2013-03-23T15:34:22+00:00] FATAL: Stacktrace dumped to /tmp/vagrant-chef-1/chef-stacktrace.out
[2013-03-23T15:34:22+00:00] FATAL: Chef::Mixin::Template::TemplateError: Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'
```